Add weighted() function

[jukent]

One card on the Trello board is to add weighted.mean() method to the Collection object. On the Xarray Documentation I see that we have 4 similar functions (weighted(), weighted.mean(), weighted.sum(), and weighted.sum_of_weights()).

In practice it seems that mean() is just called on an already weighted object. Do we want to add some sort of Decorator to a Collection object to indicate that it is weighted - or just allow someone to call mean on anything?

Also I am designing the function to go through every dataset in the collection, and create a collection of weighted datasets. I think this is the intended behavior (we're not trying to combine the different datasets here).

[jukent]

Add weighted.mean to Collection

[jukent]

I have a question about the intended use. Currently the PR #12 uses the same weights for every dataset in the collection, do we want to use different weights for different datasets?